Дисертації з теми "Soft lubrication"

The purpose of this diploma thesis is to describe the influence of input parameters (rolling speed, load, temperature, dynamic viscosity and pressure coefficient of the lubricant) on film thickness in point and elliptical contacts using optical interferometry. Regression based film thickness formula has been obtained using universal form of non-dimensional parameters of entrainment speed U, load W and material G. Equations were also complemented by the function of ellipticity k. New regression formulas for central and minimum film thickness have been compared with existing EHL thickness equations. These film thickness measurement and new regression formulas have applicability to our understanding of the performance and more effective design of lubricated gears from polymeric materials.

Dans ce manuscrit, nous étudions la réponse mécanique de films minces d'hydrogel sous plusieurs angles, incluant celui d’instabilités de la surface libre, d’indentation de la surface au moyen d’un fluide et de situations proches du contact de Hertz. Un premier chapitre préliminaire est consacré à la présentation des concepts de base utilisés dans cette thèse. Ensuite, dans une première partie, nous nous intéressons à des instabilités de surface induites par gonflement, que l'on observe à la fois sur des hydrogels gonflés et sur des films séchés. Nous analysons la formation de motifs comme le résultat d'un important gonflement anisotrope des films qui sont attachés à une surface, suivi du séchage de la surface libre de l'hydrogel de polymères, d'ores et déjà déstabilisée. Dans une deuxième partie, nous développons un modèle poroélastique pour décrire la réponse mécanique d'un hydrogel perméable soumis à un quelconque champ de pression possédant une symétrie axiale, dans un cas général. Aussi bien le cas d'une épaisseur infinie que les effets de taille finie sont étudiés et comparés. Dans une troisième partie, nous utilisons ce cadre théorique pour aborder le problème spécifique du couplage entre poroélasticité et lubrification, rencontré dans le cadre de techniques en sonde colloïdale et sans contact. Nous aboutissons théoriquement aux composantes dissipative et conservative de la force résultant du mouvement vertical d'une sphère au voisinage du substrat poroélastique. Ces résultats théoriques sont confrontés à des résultats expérimentaux de Microscopie à Force Atomique (AFM) en sonde colloïdale, obtenus sur un hydrogel épais et gonflé. Dans une dernière partie, nous mettons en évidence une succession de réponses mécaniques de la part d'hydrogels gonflés, avec des expériences d'Appareil à Forces de Surface (SFA). Partant d'un régime dénué d'interactions entre la sonde et le gel, la surface de l'hydrogel subit d'abord une faible déformation, dans un régime en lubrification. Enfin, nous montrons qu'à température ambiante la contrainte mécanique imposée déclenche par déshydratation la transition vitreuse du polymère. Dans l’ensemble, les résultats obtenus montrent que la réponse poroélastique est caractérisée par une transition dans le temps allant d’un comportement purement élastique et incompressible à un comportement purement élastique et compressible
In this manuscript, we study the mechanical response of hydrogel thin films from different perspectives, including free-surface instability, fluid-mediated surface indentation and Hertz-like contact situations. A first, preliminary Chapter is deduced to the introduction of basic concepts used is this thesis. Then, in a first part, we focus on swelling-induced surface instabilities that are observed on both swollen hydrogels and dried polymer films. The different observed morphologies are characterized by shape and spacing. We analyse the pattern formation as the result of an important anisotropic swelling of surface-attached films, and a subsequent drying of the already-destabilized free surface of polymer hydrogel. In a second part, we develop a poroelastic model to describe the mechanical response of a permeable hydrogel to any axially-symmetric pressure field, in a general case. Both the infinite-thickness case and finite-size effects are studied and compared. In a third part, we use the latter theoretical framework to address the specific poroelastic lubrication coupling associated with contactless colloidal-probe methods. We derive theoretically the conservative and dissipative components of the force associated with the oscillating vertical motion of a sphere close to the poroelastic substrate. We confront our theoretical results to colloidal-probe Atomic Force Microscopy (AFM) experiments performed on a thick and swollen hydrogel. In a last part, we highlight a succession of several mechanical responses of swollen hydrogels, with Surface Force Apparatus (SFA) experiments. From a regime with no gel-probe interaction, the hydrogel first undergoes a gentle deformation of its surface in a lubricated regime. Then, the indentation of the probe in a contact regime forces the expulsion of the solvent from the polymer matrix. We finally show that, at room temperature, the imposed mechanical load triggers the dehydration-induced glass transition of the polymer. Overall, our results show that the poroelastic response is characterized by a transition in time from a purely elastic and incompressible behaviour to a purely elastic and compressible one

Thesis (M.S.)--West Virginia University, 2001.
Title from document title page. Document formatted into pages; contains xiv, 160 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 120-122).

Les enjeux écologiques liés au réchauffement climatique, et plus généralement la lutte contre la pollution, ont occasionné une révolution sans précédent dans le domaine des transports. De nombreuses recherches portant sur l’identification de voies d’amélioration du rendement mécanique des moteurs à combustion interne ont été menées au cours de ces dernières décennies. Dans les moteurs Diesel, le contact Segments-Piston-Chemise (SPC) représente à lui seul près de 40 % des pertes d’énergie par frottement mécanique totales du moteur. Ce travail de thèse s’inscrit dans le cadre général de la lubrification des moteurs Diesel en présence de suies et s’intéresse plus particulièrement au poste SPC pour des contacts Diamond-Like Carbon (DLC) lubrifiés. Ce travail de thèse s’appuie sur des outils de tribométrie originaux pour reproduire les cinématiques particulières des contacts impliqués au niveau de la segmentation. Cette thèse s’attache à identifier l’influence d’un lubrifiant vieilli en fonctionnement sur les mécanismes de lubrification et les mécanismes de frottement associés de couches minces dures de type DLC, en balayant l’ensemble des régimes de lubrification pour des conditions stationnaires et transitoires. Les revêtements DLC développés dans le cadre de ce travail de thèse ont permis de diminuer significativement le frottement limite en conditions stationnaires et transitoires. La déstructuration du lubrifiant via la formation d’agrégats, générés par le passage des suies, ou par une annulation temporaire de la vitesse d’entraînement représentative des cinématiques de contact observées en zone SPC, sont gouvernés par le triptyque, lubrifiant, surface et cinématique de contact. Enfin, l’analyse de la réponse tribologique de l’interface lubrifiée en conditions stationnaires et transitoires permet à la modélisation théorique du frottement lors d’un cycle complet de glissement à vitesses variables
Ecological issues related to global warming, and more generally the reduction of pollution, have lead to a major revolution in the field of transport. Considerable research work has been carried out during the past decades in order to improve the mechanical efficiency of internal combustion engines. In Diesel engines, almost 40 % of total engine energy losses due to mechanical friction occur in the Piston rings-Piston-Cylinder contact (PPC). The overall framework of this PhD thesis is Diesel engine lubrication in presence of soot and this work focuses more particularly on Diamond-Like Carbon (DLC) lubricated contacts for PPC region. Unique tribometry tools are used to reproduce the particular contact kinematics involved in the piston assembly. This thesis aims to identify the influence of an aged lubricant on the lubrication and friction mechanisms of DLC hard coatings for all lubrication regimes in steady-state and transient conditions. DLC coatings developed during this thesis significantly reduce the boundary friction in steady-state and transient conditions. The lubricant destructuring due to aggregate formation, generated by the passage of soot, or by a temporary vanishing of the entrainment speed, are governed by the triplet, lubricant, surface and contact kinematics. Finally, the analysis of the tribological response of the lubricated interface in steady-state and transient conditions leads to the theoretical modeling of the friction during a complete cycle of sliding at variable velocities

碩士
國立成功大學
材料科學及工程學系
103
A soft elastohydrodynamic lubrication (EHL) model for a thin layer of fluid separating the transversely isotropic pleural surface from a rigid cylinder with an initial sinusoidal shape is presented. We consider the fluid viscous shear stress and discuss the fluid pressure, fluid thickness distribution, elastic deformation of pleura and coefficient of friction for various independent variables: Young’s modulus, Poisson’s ratio, viscosity, velocity and normal load. A finite element method (FEM) is utilized to solve the modified Reynolds equation, elasticity deformation equation and force balance equation simultaneously on a line contact soft EHL problem. The simulation results reveal that coefficient of friction is strongly dependent on normal load and velocity moderately dependent on the pleural viscosity and axial Young’s modulus, and weakly dependent on transverse Young’s modulus, transverse Poisson’s ratio and axial Poisson’s ratio. These characteristics are helpful to further investigate the biomechanical interactions between pleural surfaces with multilayer and viscoelastic properties.

碩士
國立成功大學
材料科學及工程學系
107
In this study, soft-EHL is analyzed with finite length line contact. Due to the consideration of the effect of finite length on the cylindrical roller edge, a three-dimensional model is used for simulation. The distribution of pressure, film thickness and deformation of unmodified cylindrical shape is also discussed, and compare it to the cylindrical roller after considering the round corner to observe the phenomenon of stress concentration between these two. Then the effects of different cylindrical radii on the distribution of pressure, film thickness and deformation are discussed.

碩士
國立成功大學
奈米科技暨微系統工程研究所
100
This article focuses on the elastohydrodynamic lubrication at cornea and contact lenses in biological system. When human wear contact lens, the upper eyelid will blink. In this process, the contact lens will contact with ocular surface, then the contact area will cover by the tear film which can prevent our cornea from being hurt by the direct contact with the contact lens, in order to protect our visual acuity. This mechanism is similar to the mechanical sliding bearings. According to anisotropic and incompressibility of the soft biological tissues, we discuss the special forms of anisotropic which transversely isotropic cornea model is included. In this model we have the same Young’s modulus in the xy plane but different in z. We discuss some reasons that may impact the comfort of our eyes, such as the lens material, the sliding velocity between cornea and contact lens, the load from the eyelid to the contact lens or the curvature radius of the contact lens and so on. Based on the elastohydrodynamic lubrication theory including Hertz contact theory, Reynolds equation, linear elasticity equation and the load balance equation. We can calculate some data through the finite element methods like the pressure distribution on the liquid film between two surfaces, the thickness of the liquid film, the elastic deformation of the lens and the ocular, friction and the friction coefficient. From our simulation, we know that when we wear the lens, the pressure distribution on PMMA is bigger and more centralized than on HEMA and then the liquid will be thinner because of the greater sliding. The reasons that cause the change on the ocular surface pressure distribution and the deformation are the sliding velocity between the lens and the cornea, different external load and curvature radius. These results can give ophthalmologists, optometrists and contact lens designers some references in the future.

碩士
國立成功大學
材料科學及工程學系
107
With the advancement of medical technology, technologies such as AI-assisted surgery and surgical simulation have developed rapidly. The relationship between force and displacement of the surgical procedure is especially important for the tactile and visual aspects of surgical applications. In this study, FEM is used to establish a transient squeeze soft-EHL system. The Reynolds equation coupled hyperelastic material model and the load balance equation were used to investigate different hyperelastic parameters, thickness and load conditions, by giving a constant load to a rigid ball which squeezes finite thickness of the hyperelastic flats. Considering the fluid can withstand the load, using this model as an indentation simulator, discuss the effect of fluid on force and displacement. The gray matter and white matter of brain tissue which is extremely soft and requires a small load during surgery are selected as our studying objects. The simulation results reveal that the assumptions of the EHL model should be corrected if soft material produced large deformation. The difference between the force and the displacement of the dry contact of hyperelastic model and soft-EHL increases as the load increases. The softer gray matter has a larger deformation than the white matter, and the difference in deformation between the two increases with thickness. When the amount of deformation is fixed, it can be found that as the thickness increases, the load required for the white matter is larger than that of the gray matter.

Dissertação de Mestrado Conjunto Europeu em Tribologia de Superficies e Interfaces apresentada à Faculdade de Ciências e Tecnologia
This project is motivated from now days trends in the automotive industry to increase fuel efficiency and fulfill environmental standards. To do so the increase of soot (carbon black particles) in the Internal Combustion Engines has become a big issue, challenging the environmental impact of disposing oil and overall maintenance cost of running a vehicle. Diamond Like Carbon (DLC) thin films are widely used for protection of components under high load from wear and/to reduce friction loses in combustion engines. To furthermore increase the properties of DLC, using Magnetron Sputtering (DCMS), standard steel balls for the 4 Ball Tester were coated with DLC doped with tungsten (W-DLC) and DLC doped with silver (Ag-DLC). In basic, the interaction among soot/W-DLC coated steel surfaces and soot/Ag-DLC coated steel surfaces as a possible solution to the soot wear problems and further development of soot wear resistant lubricants has been investigated. Before the test, the type of contact of the tribopair has been studied. Friction and wear properties of W-DLC and Ag-DLC have been tested in boundary lubrication conditions using a 4 Ball Test. To check the coating efficacity and its interaction with the base oil(Ester Base Oil), additive (Glycerol Monooleate) and soot (Carbon Black), Optical Microscope for wear scar images, 3D Profilometry to obtain the 3D profile of the wear scar and Raman Spectroscopy to check on the coating structure have been applied. Test results showed the benefit of the W-DLC/Steel and Ag-DLC/Steel contact over the Steel/Steel contact.
This project is motivated from now days trends in the automotive industry to increase fuel efficiency and fulfill environmental standards. To do so the increase of soot (carbon black particles) in the Internal Combustion Engines has become a big issue, challenging the environmental impact of disposing oil and overall maintenance cost of running a vehicle. Diamond Like Carbon (DLC) thin films are widely used for protection of components under high load from wear and/to reduce friction loses in combustion engines. To furthermore increase the properties of DLC, using Magnetron Sputtering (DCMS), standard steel balls for the 4 Ball Tester were coated with DLC doped with tungsten (W-DLC) and DLC doped with silver (Ag-DLC). In basic, the interaction among soot/W-DLC coated steel surfaces and soot/Ag-DLC coated steel surfaces as a possible solution to the soot wear problems and further development of soot wear resistant lubricants has been investigated. Before the test, the type of contact of the tribopair has been studied. Friction and wear properties of W-DLC and Ag-DLC have been tested in boundary lubrication conditions using a 4 Ball Test. To check the coating efficacity and its interaction with the base oil(Ester Base Oil), additive (Glycerol Monooleate) and soot (Carbon Black), Optical Microscope for wear scar images, 3D Profilometry to obtain the 3D profile of the wear scar and Raman Spectroscopy to check on the coating structure have been applied. Test results showed the benefit of the W-DLC/Steel and Ag-DLC/Steel contact over the Steel/Steel contact.